Benzylation of aromatic compounds catalyzed by 3-methyl-1-sulfonic acid imidazolium tetrachloroaluminate and silica sulfuric acid under mild conditions

Article abstract of DOI:10.1055/s-0033-1338386

In this work, efficient procedures for benzylation of a range of aromatic compounds by benzyl acetate in the presence of catalytic amounts of 3-methyl-1-sulfonic acid imidazolium tetrachloroaluminate ([Msim]AlCl ₄) or silica sulfuric acid (SSA) under mild conditions are described. Simple methodology, easy workup procedure, clean reaction and reusability of the catalyst are some advantages of this work. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1338386

LETTER
▌1113
letter
Benzylation of Aromatic Compounds Catalyzed by 3-Methyl-1-sulfonic Acid
Imidazolium Tetrachloroaluminate and Silica Sulfuric Acid under Mild
Conditions
Benzylation of Aromatic Compounds
Mohammad Ali Zolfigol,*a Hooshang Vahedi,b Saeid Azimi,b Ahmad Reza Moosavi-Zarec
a
Faculty of Chemistry, Bu-Ali Sina University, Hamedan 6517838683, Iran
Fax +98(811)8257407; E-mail: mzolfigol@yahoo.com
b
Department of Chemistry, Payame Noor University, PO Box 19395-4697, Teheran, Iran
c
Department of Chemistry, University of Sayyed Jamaleddin Asadabadi, Asadabad 6541835583, Iran
Received: 01.02.2013; Accepted after revision: 24.03.2013
–
AlCl4
+
Abstract: In this work, efficient procedures for benzylation of a
range of aromatic compounds by benzyl acetate in the presence of
catalytic amounts of 3-methyl-1-sulfonic acid imidazolium tetra-
chloroaluminate ([Msim]AlCl4) or silica sulfuric acid (SSA) under
mild conditions are described. Simple methodology, easy workup
procedure, clean reaction and reusability of the catalyst are some
advantages of this work.
N
N
Me
SO3H
Figure 1
Herein, we describe the use of SSA and [Msim]AlCl4 as
catalysts for benzylation of a range of aromatic com-
pounds under mild conditions to give diarylmethanes, and
compare their catalytic activity in this reaction (Scheme
1). The promising aspects of this methodology are its ef-
ficiency, high yield, cleaner reaction profile, and simplic-
ity, rendering it an attractive process for the preparation of
diarylmethanes.
Key words: benzylation, benzyl acetate, 3-ethyl-1-sulfonic acid
imidazolium tetrachloroaluminate, silica sulfuric acid
Diarylmethanes are an interesting group of bioactive com-
pounds, and are also very important and useful in organic
synthesis.1–3 Catalytic systems used for the benzylation of
aromatic compounds include HfO2–HfCl4,3 aliquat 336,4
phosphomolybdic acid supported silica gel,5 TMSOTf,6
organozinc and organoboron derivatives, HCl, H2SO4,
MeSO3H, HOTf and HNTf2.7 However, some of these
procedures suffer from various drawbacks, such as com-
plex workup and purification, production of significant
amounts of waste materials, use of strongly acidic condi-
tions, and occurrence of side reactions with poor yields.
To optimize the reaction conditions, we selected the reac-
tion of anisole and benzyl acetate as a model reaction
(Scheme 2).
We observed that the best results were obtained using
5 mol% of [Msim]AlCl4 at 70 °C or 7 mol% of SSA at
80 °C (Table 1).
In a further study, the model reaction was tested with
[Msim]AlCl in several solvents; the corresponding results
are displayed in Table 2, indicating that the best result was
obtained under solvent-free conditions.
In recent years, silica sulfuric acid (SSA), as one of the
more important solid acid catalysts,8–12 and 3-methyl-1-
sulfonic
acid
imidazolium
tetrachloroaluminate
Subsequently we examined various aromatic substrates
with benzyl acetate as electrophile under solvent-free con-
ditions and with solvent using SSA or [Msim]AlCl4 as
catalyst (Table 3). In certain instances, reaction occurred
([Msim]AlCl4), as a new sulfonic acid functionalized im-
idazolium salts (SAFIS) with interesting applications,
have been reported (Figure 1).13–15
method a: [Msim]AlCl4 (5 mol%), 70 °C
method b: SSA (7 mol%), 80 °C
+
ArH
+
AcOH
Ar
OAc
Scheme 1 The preparation of diarylmethanes catalyzed by [Msim]AlCl4 (method a) and SSA (method b)
OMe
OMe
[Msim]AlCl4 or SSA
+
+
AcOH
OAc
60–90 °C
Scheme 2 Benzylation of anisole as model reaction
SYNLETT 2013, 24, 1113–1116
Advanced online publication: 16.04.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1338386; Art ID: ST-2013-D0110-L
© Georg Thieme Verlag Stuttgart · New York

1114
M. A. Zolfigol et al.
LETTER
Table 1 Effect of Different Amounts of the Catalyst and Tempera-
ture on the Reaction of Anisole with Benzyl Acetate under Solvent-
Free Conditions
Table 2 Reaction of Anisole with Benzyl Acetate Using
[Msim]AlCl4 (5 mol%) in Different Solvents at Reflux
Entry
Solvent
Time (min)
Yielda (%)
Catalyst
Catalyst
(mol%)
Temp
(°C)
Time
(min)
Yielda
(%)
1
EtOH
120
120
90
46
34
15
75
15
85
2
MeOH
MeCN
CH2Cl2
MeCOOEt
–
[Msim]AlCl4
[Msim]AlCl4
[Msim]AlCl4
[Msim]AlCl4
[Msim]AlCl4
[Msim]AlCl4
[Msim]AlCl4
SSA
1
3
70
70
70
70
70
60
80
80
80
80
80
80
70
90
90
90
48
60
85
85
85
40
85
51
60
74
83
83
69
83
3
4
60
5
60
5
100
60
7
60
6b
10
5
60
a Isolated yield.
120
60
b Reaction was carried out at 70 °C.
5
1
120
120
90
with electron-donating substituents gave high yields of
benzylated products under mild conditions.
SSA
3
Mendoza et. al have reported benzylation of certain aro-
matic substrates using HOTf as a Brønsted acidic catalyst
but indole was found to react poorly with the acid catalyst,
leading to decomposition products and unreacted indole.7
However, with our protocol, 3-benzyl indole was obtained
in good yield using [Msim]AlCl4 or SSA as catalyst (Ta-
ble 3, entry 6). Generally, our protocol is more efficient
than previous methods;16 with substrates possessing addi-
tional electron-donating groups, the yield increased and
reaction time decreased (Table 3, entries 1 and 2) and,
with phenol as substrate, the p-benzylphenol was pro-
SSA
5
SSA
7
60
SSA
10
7
60
SSA
90
SSA
7
60
a Isolated yield.
in the absence of solvent (Table 3 entries 1–3, 7 and 8) and duced rather than the o-benzylphenol. Furthermore, with
in others, CH2Cl2 was the reaction solvent. Substrates indole, trimerization was not observed.17
Table 3 Synthesis of Diarylmethanes Using SSA and [Msim]AlCl4 as Catalyst
Entry
Nucleophile
Main product
Time (h)/
Time (h)/ Time (h)/
Yielda (%)
Yielda (%) Yielda,b (%)
{[Msim]AlCl4} (SSA)
OMe
OMe
1
1/93
1/89
1/867
OMe
OMe
OMe
OMe
1/2321
2
3
1/85
1/83
5/80.318
0.5/2321
OH
1.5/86
1.5/82
OH
0.58/3220
6/6519
4c
2/95
2/89
2/89
2/80
OH
OH
5c
–
OH
OH
Synlett 2013, 24, 1113–1116
© Georg Thieme Verlag Stuttgart · New York

LETTER
Benzylation of Aromatic Compounds
1115
Table 3 Synthesis of Diarylmethanes Using SSA and [Msim]AlCl4 as Catalyst (continued)
Entry
Nucleophile
Main product
Time (h)/
Time (h)/ Time (h)/
Yielda (%) Yielda,b (%)
Yielda (%)
{[Msim]AlCl4} (SSA)
6c
3/87
3/80
18/6916
N
H
N
H
7
8
2/86
2/81
3/87
40/633
40/873
2.5/92
9c,d
3/95
4/90
–
–
HO
OH
HO
HO
OH
10c,e
3.5/90
4.5/86
OH
HO
OH
a Isolated yield.
b Literature results compared with the presented results.
c These reactions were carried out in CH2Cl2 under reflux.
d Ratio of benzyl acetate to nucleophile was 4:1.
e Ratio of benzyl acetate to nucleophile was 2:1.
Table 5 Reaction of Anisole and Benzyl Acetate in the Presence of
Recycled SSA under Solvent-Free Conditions at 80 °C
The recyclability of the catalyst was also investigated with
the reaction of anisole and benzyl acetate as a model reac-
tion. The reaction mixture was extracted with CH2Cl2 to
separate products form the [Msim]AlCl4 or with ethanol
to separate from the SSA. Subsequently, the catalyst was
employed for a second cycle reaction. We observed that
the catalytic activity of [Msim]AlCl4 (Table 4) and SSA
(Table 5) was retained, within the limits of experimental
error, for five and four successive runs, respectively.
Entry
Cycle
Time (min)
Yielda (%)
1
1st run
2nd run
3rd run
4th run
60
85
83
75
68
60
2
3
100
130
4
Table 4 Reaction of Anisole and Benzyl Acetate in the Presence of
Recycled [Msim]AlCl4 under Solvent-Free Conditions at 70 °C
a Isolated yield.
Entry
Cycle
Time (min)
Yielda (%)
In summary, we have reported two new procedures22 for
the preparation of diarylmethanes by the benzylation of a
range of aromatic compounds using 3-methyl-1-sulfonic
acid imidazolium tetrachloroaluminate {[Msim]AlCl4} or
silica sulfuric acid (SSA) as heterogeneous catalysts with
high yields under mild conditions.
1
1st run
2nd run
3rd run
4th run
5th run
60
80
85
80
73
67
61
2
3
90
4
115
120
5
Acknowledgement
a Isolated yield.
The authors gratefully acknowledge partial support of this work by
the Research Affairs Office of Bu-Ali Sina University (Grant num-
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1113–1116

1116
M. A. Zolfigol et al.
LETTER
ber 32-1716: Development of Chemical Methods, Reagents and
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General Procedure for the Synthesis of Diarylmethanes
Using SSA: To a mixture of aromatic substrate (1 mmol),
and benzyl acetate (1 mmol), silica sulfuric acid (0.054 g, 7
mol%) was added and the mixture was heated to 80 °C for
the appropriate time (Table 3). After completion of the
reaction, as monitored by TLC, the reaction mixture was
cooled to r.t. Absolute EtOH (2–5 mL) was added and the
mixture was filtered to separate the catalyst. The pure
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Synlett 2013, 24, 1113–1116
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